A redshift-dependent colour–luminosity relation in Type 1a supernovae

Abstract

Type 1a supernova magnitudes are used to fit cosmological parameters under the assumption that the model will fit the observed redshift dependence. We test this assumption with the Union 2.1 compilation of 580 sources. Several independent tests find that the existing model fails to account for a significant correlation of supernova colour and redshift. The correlation of magnitude residuals relative to the Λ CDM model and colour × redshift has a significance equivalent to 13 standard deviations, as evaluated by randomly shuffling the data. Extending the existing B − V colour correction to a relation linear in redshift improves the goodness of fit χ2 by more than 50 units, an equivalent 7σ significance, while adding only one parameter. The colour−redshift correlation is quite robust, cannot be attributed to outliers and passes several tests of consistency. We review previous hints of redshift dependence in colour parameters found in bin-by-bin fits interpreted as parameter bias. We show that neither the bias nor the change Δχ2 of our study can be explained by those effects. The previously known relation that bluer supernovae have larger absolute luminosity tends to empirically flatten out with increasing redshift. The best-fitting cosmological dark energy density parameter is revised from ΩΛ = 0.71 ± 0.02 to ΩΛ = 0.74 ± 0.02 assuming a flat universe. One possible physical interpretation is that supernovae or their environments evolve significantly with increasing redshift.